This invention relates to a method of preparing high purity silicon. More specifically, this invention relates to a method of preparing high purity silicon from Na.sub.2 SiF.sub.6 (sodium fluosilicate).
The production of large single crystals of silicon, from which single crystal silicon solar cells are manufactured requires large quantities of high purity silicon. The crystals are pulled by the Czochralski method from semiconductor grade silicon, and then cut into wafers of appropriate thickness which are fabricated into solar cells. Semiconductor grade silicon is presently prepared by reducing purified silicon tetrachloride or trichlorosilane with purified hydrogen.
Research is also being conducted into other methods for preparing silicon for solar cells from other forms of silicon which may be less expensive. However, by which ever method the solar cells are prepared, the cost of high purity silicon to produce them is high and may go higher due to accelerated growth in the semiconductor and solar cell industry. Furthermore, the increased demand may bring about a shortage of suitably pure silicon.
One readily available source of silicon is sodium fluosilicate (Na.sub.2 SiF.sub.6) which is a by-product of the phosphate fertilizer industry. It is estimated that 500,000 metric tons of this material are produced each year. A process which could reduce this compound to high purity silicon suitable for use in the semiconductor industry or for preparing single crystals of silicon would be expected to find wide utilization.
One method which has been developed for the reduction of sodium fluosilicate is the metallothermic or "bomb" reduction. In this method of reduction, the compound to be reduced is mixed with one or more reducing agents and the resulting mixture is ignited. Ideally, the reaction continues spontaneously with complete oxidation of the reducing elements, forming a fluid slag while the reduced metal is obtained as a compact uniform regulus. In order for this technique to be successful, the heat of reaction must be sufficiently large so that under adiabatic conditions both the silicon and slag products are molten with separation of silicon and slag into two separate aggregates.
Compounds which will reduce fluosilicate include any of the alkali, alkaline earth metals, magnesium and aluminum. However, the choice is limited because of costs and other considerations, such as recovery of the metal reductant from the silicon if an alloy is produced. If alloying of the reductant with silicon occurs, either intentionally or unintentionally, the reductant must be sufficiently volatile to be removed by heating the alloy under vacuum at some reasonable temperature, preferably below 1000.degree. C. Thus the most economical, suitable reducing agents are sodium and magnesium.
The use of either sodium or magnesium alone as a reductant for Na.sub.2 SiF.sub.6 will not supply sufficient heat to melt the reaction product to form a compact regulus so that a thermal booster must be added to generate the extra heat required. Sulfur has been found to be the most suitable booster for use with sodium, however, the reaction requires 3.12 mol of sulfur in addition to 10.20 mol of sodium for every one mol of potential silicon produced making the sodium-sulfur system undesirable and uneconomical. Boosters which have been considered for use with magnesium include Al and KClO.sub.3, KClO.sub.3 alone and sulfur. However, the combination of magnesium and any of these boosters provided a silicon yield which was no higher than about 66%. Furthermore, the purity of the resulting silicon was not sufficiently high to be used for preparing single crystals due to impurities in the fluosilicate, the magnesium and the booster.